The present invention relates to solid state polymerization of low molecular weight polyester particles, and, more particularly, to a method for increasing the rate of polymerization of such particles under solid state processing conditions whereby the progress of polymerization is preferred over the progress of crystallization for a given mass of particles. As a result, the reaction time in the solid state required to reach a selected polymer molecular weight is substantially reduced.
It is known to produce high viscosity, i.e., high molecular weight, polyester polymers in the solid phase at temperatures below the melting point of the low molecular weight starting pre-polymer particles and under an inert gas blanket or vacuum. According to U.S. Pat. No. 4,064,112, the is tendency of the particles to agglomerate due to stickiness during solid state polymerization (SSP) can be reduced and even eliminated if the solid state polymerization is preceded by a crystallization step utilizing temperatures which are higher than, or equal to, the temperature at which the solid state polymerization is carried out.
U.S. Pat. No. 5,540,868 describes a process for forming uniform, crystalline low molecular weight polyester particles in size ranges suitable for mass handling. Such particles, or pellets, which have a degree of crystallinity greater than about 15%, are especially advantageous for use as a pre-polymer feestock for solid state polymerization. The particles can be introduced into the SSP reactor within 10 minutes after the particles are formed. Alternatively, it is also possible to store the pellets for an indefinite period of time before feeding them to the SSP reactor.
Despite the ability to produce uniform, crystalline low molecular weight polyester particles which can be directly polymerized in the solid state without further crystallization or annealing, there is a need to reduce the generally long processing time, e.g., up to 30 hours or longer, which is typically required using conventionally known SSP techniques to increase the molecular weight of these particles to the minimum requisite level for commercial use.
The present invention resides in the discovery that the rate of polymerization of low molecular weight polyester pre-polymer particles under solid state processing conditions can be substantially increased by observing specific parameters with respect to particle heating, i.e., conditioning. By following the processing conditions in the sequence described according to the invention, the progress of polymerization for low molecular weight polyester pre-polymer particles in the solid state is preferred over the progress of crystallization within a given mass of particles. The polymerization reaction time for a given low molecular weight pre-polymer to reach a desired higher molecular weight range is substantially reduced.
The present invention is an improved process for polymerizing low molecular weight polyester prepolymers in the solid state which comprises:
(a) contacting the particles with a heat-transfer medium to heat the particles to a temperature in the range of from about 205xc2x0 C. to 240xc2x0 C. in less than 10 minutes with the result that the time at which the particles are within the temperature range of 100xc2x0 C. to 205xc2x0 C. is minimized,
(b) maintaining the temperature achieved for the particles in step (a) for at least one hour by subjecting the particles to a continuously moving stream of inert gas at elevated temperature and at a mass of gas to mass of solids ratio which can be as low as 0.05 whereby the progress of polymerization of said particles is preferred over the progress of crystallization, and then
(c) subjecting the particles to a countercurrently moving stream of heated inert gas at a mass of inert gas to mass of solids ratio which is less than the mass ratio maintained in step (b), but the total mass of inert gas to mass of solids ratio for step (a) and step (b) is equal to or greater than 0.6.
In a preferred embodiment of the invention the low molecular polyester particles are essentially-crystalline, meaning that they have a crystallinity content greater than about 15%, which corresponds, for polyethylene terephthalate (PET), to a density greater than about 1.36 g/cc. In addition, the heat transfer medium is a stream of inert gas at elevated temperature, and the particles are contacted in step (a) at a mass of gas to mass of solids ratio in the range of from 4:1 to 15:1 to effect a rapid rise in temperature whereby the polyester particles reach a temperature of at least 205xc2x0 C. in less than 10 minutes.